1. Field of the Invention
The present invention relates to a projection system, and particularly relates to a liquid crystal projection system that utilizes three liquid crystal panels.
2. Description of Prior Art
In recent years, in term of the number of liquid crystal panels to be used for displaying an input image signal, liquid crystal projection systems have been developed from a single-panel type to a three-panel type. A liquid crystal projection system with three LCoS (Liquid Crystal on Silicon) panels comparatively offers high resolution, high color fidelity and high brightness images.
A conventional liquid crystal projection system with three LCoS panels is disclosed in U.S. Pat. No. 6,454,416, in which additional optical elements are provided between each of the two PBSs (Polarization Beam Splitters) and the dichroic prism thereof. These additional optical elements include a selected-wavelength phase plate for rotating the polarization of a corresponding incident primary-color light beam by 90°, and a glass plate for equalizing the optical path lengths of the light beams from the three panels. U.S. Pat. No. 6,819,497 also discloses a conventional three-panel liquid crystal projection system, in which four PBSs are employed and a color selective polarizer or a glass plate is also disposed between opposing PBSs.
It is known that, for a liquid crystal projection system employing three LCoS panels, high assembly precision is required. In order to make the images formed on the three LCoS panels to correctly overlap with each other so as to obtain a high-resolution image, each component prism must be made reliable, which mostly depends on the cementing technology for prism. Therefore, the cementing between each PBS and the dichroic prism must be strictly controlled. However, with the technical development of the LCoS panel, the dimension of pixels on the LCoS panel has been increasingly reduced. Consequently, the cementing between each PBS and the dichroic prism must be further strictly controlled to be within an acceptable tolerance. It is clear that, in the conventional designs as discussed above, since additional optical elements are further required to be cemented between the PBSs and the dichroic prism, the difficulty in the prism assembly is correspondingly increased, which is not desired and which increases the manufacturing cost.
In addition, the increase of system cementing interfaces between the PBSs, the dichroic prism and the additional optical elements will decrease the display contrast and hence adversely affect the image quality. This is because the light absorption of the prisms and the cementing material during work will result in temperature rise. Since the various components are different in temperature and in thermal expansion coefficient, deformation stress will occur at the cementing interfaces and the soft cementing materials will exhibit stress birefringence consequently. As a result, the display contrast of the output image will be adversely decreased after the image light beams reflected from the three LCoS panels pass the cementing materials.
As discussed above, in conventional liquid crystal projection systems with three LCoS panels, additional optical elements are generally required to be disposed between adjacent PBSs or between the PBS and the dichroic prism. This complicates the assembly process and decreases the image contrast. Hence, an improved three-panel liquid crystal projection system is desired to address the above problems that are encountered in the prior art.